Defined in 3GPP TS36.213, the uplink (UL) power control of the LTE is a combination of a open loop mechanism and a closed loop mechanism. Wherein, the open loop mechanism means that the transmit power of a user equipment (UE) is depending on an estimation of downlink (DL) path loss, while the closed loop mechanism means that the network may additionally control the transmit power of a UE directly by an explicit power control command that is transmitted in downlink. The open loop power control (OLPC) is mainly responsible for a rough adjustment of the transmit power of a UE, and it mainly compensates the slow change of path loss to obtain a certain average received signal power for all UE. While the closed loop power control (CLPC) is mainly used for a UE-specific adjustment of the power configuration, it can eliminate the influence of quick change of channels, and match or be as close as possible the receiving SINR to further optimize the overall network performance.
According to the quantity of resource blocks scheduled for PUSCH transmission, the transmission power (i.e. uplink power) in each subframe is deduced from a semi-static operation point and a dynamic bias. In 3GPP, the power control formula of PUSCH transmission is defined by the following expression:PT=min{Pmax,10·log10(M)+P0+α·PLDL+ΔMCS+δ}  (1)
Wherein, PT is the transmit power of a given subframe, Pmax is the allowed maximum transmit power of the UE, e.g. 23 dBm, M is the bandwidth of PUSCH measured by the quantity of physical resource blocks (PRB), PLDL is the downlink path loss measured by the UE.
And, wherein, P0 is a semi-static basic level, α is a compensation factor of open loop path loss, which depends on many factors including inter-cell interferences and cell loads.
Besides, ΔMCS is a component related to modulating coding scheme (MCS), and it reflects the fact that different modulation schemes and coding rates need different SINRs. δ is a UE-specific adjustment value which is indicated by an explicit TPC command, and it's a UE-specific CLPC corrective value at a semi-static operation point.
The above introduces the power control schemes in 3GPP. The following describes co-channel interferences in a heterogeneous network, and introduces shortcomings existed in current technology.
As shown in FIG. 1, there are two cases where serious UL co-channel interference may occur in heterogeneous networks. First, at an edge of a macro cell, a macro-UE (MUE) transmits with a high power to an eNB in order to overcome large path loss between the MUE and the eNB. In this case, if an Pico eNB (or RRH) RRH2 locates at the edge of the macro-cell, a pico-UE (PUE) associated with RRH2 suffers from a serious interference of MUE which transmit UL signals with the high power. On the other hand, in the case that an RRH locates close to a macro-eNB, such as RRH1, the UL signals transmitted by a PUE associated with RRH1 may become a serious interference to MUE which transmit UL signals to the macro eNB.
In order to coordinate the co-channel interferences in the above different cases, the UL power controls of UEs associated with different RRHs should have different design targets. E.g., for the RRH locating at the edge of a cell, the UE associated with them should use higher transmit power to overcome the interference from a MUE. In another example, for the RRH near macro-eNB, the UE associated with them should use lower transmit power to avoid serious interference to a MUE. Thus, the adaptive adjustment of power control is favorable, such as adjustment depending on the location of a RRH relative to a macro-eNB. (J. Gora, K. I. Pedersen, A. Szufarska and S. Strzyz, “Cell-specific uplink power control for heterogeneous networks in LTE”, IEEE VTC2010-Fall, Canada Ottawa, September 2010)
For UL CoMP, due to the coordinated processing among different reception points, the conventional power control parameters need to be modified to take the specific UL CoMP algorithms into account. Especially in the scenarios that the cooperation areas of UL CoMP are different for different UEs, the conventional OLPC parameters need to be reconsidered to fully utilize the CoMP gains.
Based on the above analysis, the common configuration of transmit power is suboptimal in heterogeneous networks, especially for CoMP. The transmit power can be adjusted to achieve better macro-cell performance at the cost of performance at pico-cells and vice versa. An appropriate configuration of transmit power may be selected according to an equivalence between performances of the cells of the two tiers.
In the current agreement, scenario 4 is one of important scenarios to be investigated. Scenario 4 is a heterogeneous network with low power RRHs within the macro-cell coverage, wherein, the transmission/reception points created by the RRHs have the same cell IDs as that of the macro-eNB.
With the current UL power control methodology as defined in 3GPP, it can not realize respective configuration of power control parameters for the UEs associated with different RRHs and the macro-eNB, since the base level P0 and the path-loss compensation factor α are cell-specific parameters and broadcasted by the RRC signaling. It means that all UEs within the macro-cell coverage share a common configuration of power control parameters regardless of different points to which they are actually associated with. Therefore, the performance of the two tier networks in this type of heterogeneous network deployment is not optimized, due to the serious influences potentially caused by the above mismatch configurations of power control.
In addition, it is obvious that the coordinated processing can not achieve full UL CoMP gains in heterogeneous networks with shared cell-ID since it doesn't differentiate the power settings for CoMP UEs and non-CoMP UEs.